This is - technically - a duplicate post, but I wanted to get some additional exposure so I posted this in new tread.

My 2002 zx3 is having issues and "experts" have replaced bunch of parts (problems started almost 2 years ago and got progressively worse, now it really stutters almost every ride). Problem code is always P0304 (not always showing despite of noticeable miss). 1st "fix" was when I turned it in to do timing belt (~120K miles), asked them to check for the problem - really acceptable at the time but I did now want to do timing and water pump only to junk the car little while later. Could not find nothing, actually suggested replacing the MAF at ridiculous price (so I did it myself - had connector kinda cracked so OK to swap it). Car ran fine for a month or two and then issue again. Did fuel filter (this replacement might had preceded the whole problem actually), plugs, wires and even coil (ouch, gets pricey). Did also egr/dpfe sensor (had that at almost new car, similar effects while driving but not across the whole range of speed/rpm, cleaned up egr valve). Problem continued. And Cel is always P0304 - cylinder 4 misfire. Pulled injectors, tested all - seemed ok, switched 1 and 4 around. No change. Damn. Took it back to the same shop and asked for diagnostics - compression, scanner etc (whatever they had, small/medium shop, not a Ford dealer service center). They diag - injectors (right) and intake manifold gasket. Really did not like injectors idea (I had told them about all test/replacements/swaps) but I caved in. After return the car ran fine for like 2 weeks and then again the problem. Took it back to ask for warranty job. Dude took it for a test drive with me in passenger seat. Man, this fool tried to race it on city streets though the way he operated clutch made me regret letting him behind the wheel. Not only this but moron did not disengage the park brake, rear drums were smoking when he dropped me off. Told the fool to fix brakes he did. During the ride I did not feel much misfire but it's likely that if you push it like that it's really hard to notice it. After return the car was no better and all that after spending quite a bit of money (what I really hate is how difficult is to know/account for what was really done, forgot to ask for compression test readout so this is partially my own mistake). Time passed and the problem got even worse. It never stalled on me, idle is OK (not great) but driving feels like going against hurricane force gusts, acceleration suffers, fuel economy is no different from what it was. I replaced the battery (original, it was time anyway, winter forced me to), problems did not go away, my radio still goes nuts at times - I don't think this is related though. Opened ECU compartment, unplugged wires and plugged it back in, replaced fuel filter (again), fuel pump - hard to get to (will do only if have to). After driving home from work got check engine blinking, crap. Trying to gun it while it sputters does not do much. Replaced ignition coil pigtail (mid winter job, small radioshack iron - solder barely wanted to flow, did not expect that). Same issue on the test drive. Took it to another shop, asked for another diagnostics, same results - dude "drove the snot out of it", 40 miles but could not find a problem, watered my engine bad enough that spark plugs rusted. My car was having all the problems but now it stank with cigarettes. I lost all faith in mechanics (and their ability to conduct proper diagnostics). Bought an android tablet and Torque Pro app, cool - now I could run diagnostics the shop did not do/know how to interpret. Neither did I - information overload, none of data/observed issues seems unique to attribute to one thing. All I knew was that fuel pressure seems ok (it varies - never a straight line but it's not low), intake manifold pressure seems fine, fuel trim does not seem OK (long term seems to vary more than short, both run high - lean condition). O2 signal seems odd - pre cat one not always have a jumpy line (should it?) and the post cat is not always high (and will dive when missing, fuel trims will do strange things as well). Old man at the last shop did not like spark cables (Bosh) - got another replacements and motorcraft coil (autozone one had some strange resistance reading on the flat side of contacts - I could not keep it stable). Wanted to inspect the intake manifold myself so got gaskets as well. Took the thing off, checked fuel rail for crap (that would end up in the whole for injector 4 and starve the cylinder) - all clear. PCV hose kept me from taking the whole thing out of engine bay (could not disconnect it, did not wont to cut - had no spare). Cleaned up intake manifold inside, cleaned up idle air valve. Tested injector 4 wire harness - 0 resistance regardless of bending the rest of harness, did not take it completely apart - too much pain. Got a cheap endoscopic usb camera, looked int cylinders (hard to get to for real check) and intake valves. Put thing together (damn knock sensor cable/plug, what a pain). Car did the usual on the way from work. Now I'm looking for a car but really stuff is expensive, even more complicated (future repairs will be killer), too much crap but lacking substance. And colors suck (gray or silver, maybe red). And gas mileage - no progress. Stick hard to get (sure they want these future repairs). I like to keep it but it getting tough.
Now, there are some other common things with your post. Noticed recently that I could induce sputtering when playing with fuel pressure sensor (and found that I broke the clip so it won't latch securely, put zip tie to lock it in place). Then I noticed that it's actually not the FPS pigtail but the way it rocks the rest of the engine harness that causes the problems. I could push the injector wire cage/rail right above the 1st cylinder to trigger the issue. Then I pressed the plastic rail against fuel rain (so it stayed in place) and worked on the harness from other side. I don't think the issue is before the main connector next to firewall (going from ECU, right below evap(?)). What it appears is that the problem spot is at junction of main harness and FPS lead/pigtail. This will be my last thing to check replace before giving in. I hope that they have some crashed Focus at nearby junkyard (spent enough money not to go all out for replacement). If I can't source it cheaply I'll just open up the original harness and see what's in there.
Anyone has a suggestion, something that I (and bunch of mechanics missed)?
BTW, does anyone know where the knock sensor plug attaches to (it has a plastic fastener that I could not find spot for - it's hard to get to as well)?

That's a long post... lol What I got from it is that your cylinder 4 is misfiring? Is that all it's doing? (Don't feel like reading all of it again... haha sorry) Did you gap your plugs correctly? Are your plug wires firmly clipped on to your plugs and coil pack? What do your spark plugs look like? If you unplug spark plug 4 does your idle change or does the car run the same way? Are you sure number 4 is working on your coil pack correctly?

Just some things to think about and inspect, and always double check everything.

I'm thinking since I think you said you changed your coil pack like twice.... I think that's what you said in all of that rambling. jk lol It could possibly be in the plug/wiring harness that connects to the coil pack and back to wherever it comes from. (I'm not an electrician, so I have no idea about all of that stuff.) But I'm thinking if it's not your plugs or wires, or your coil pack... it has to be something in the wiring to the coil pack or ignition somewhere. (maybe a mouse got in there and chewed something it shouldn't have, lol you never know) You said your injectors are okay so I wouldn't think it could be that.

But since it's cylinder 4, I would start at the cylinder and work my way out exhausting all of the possible causes. And i'd start with the cheapest solution first. Don't want to spend too much money.

Just thought I'd throw my 2 cents in there. I'm sure you have already tried all of this stuff I've told you... but it never hurts to double and triple check. Hope you figure it out soon.

I think it would have to do something with the coil pack too I had similar problems with my svt focus also the pre cat o2 should fluctuate but the post cat o2 should look steady if not u probably have a bad cat or oxygen sensor

I'd say that if you can duplicate the problem by wiggling the harness you may have narrowed down the source of the issue.

Harness issues do NOT fall into a "common failure" category, with the exception of the "pigtails" to the coil or alternator, so I can't confirm this as a likely problem.

Testing it carefully to confirm it's an internal harness issue rather than the harness pulling on a connector with poor contact would be the next step. Only then would you want to "go inside" the harness cover to continue trying to narrow it down.

A number of members have 'peeled back" the harness cover & rewired/rearranged things to create a "cleaner" appearance, so it CAN be done. You'd want to open it up & continue the testing to see which wire or wires is causing your problem. IF you don't get lucky & find a "new" junkyard harness cheap to just swap in.

Most wiring issues happen at or near connectors, that's why I mention checking those areas first B4 diving too deep into the harness itself.

Brush your offending connectors clean, then grease, reinstall- see if that helps. If not, you might want to replace that harness. Another thing, IIRC, the 02 has a metal intake manifold, at least I think so. If not, disregard this. If you do have a metal intake manifold, run a wire from a bracket screw- maybe fuel rail hold down screw- and then take it over to the car's chassis.

Replace PCV line and the PCV. Yes, you can get it off. Try twisting on it instead of just pulling. Ford makes an improved insulated line for $20. It's worth it. Most of the problems with that vacuum line are due to heat.

If this doesn't work, then swap out your platinum or iridium plugs for standard plugs. No, that won't cure your problem, however within a few thousand miles you'll have another clue to your problem- spark plug deposits. You can't get those with plat or iridium plugs. At least not in a short time. That might help you diagnose your issue, for example, your intake manifold vacuum looked good, but you could still have a small leak like an injector washer or the manifold might be loose at the end of the manifold.

Long post was for reason - I wanted to provide the background, replying back and forth to posts takes more time that going through one long post.

Spark plugs are gapped, look fine and switching them around does not good - unlikely the source. Cables and even coils were switched as well.

O2 sensors - already have replacements but top one is pain to get to. Still, it's likely that observed signal issue are result and not the cause. Will replace them anyway.

Harness - last hope (and big problem).Today I retested all wires using multimeter (wiggled the crap out of them) but no dice. Not sure how to interpret results (the other day I definitely choked rpms when bending harness around the junction where it splits to FPS sensor). It's quite likely that the harness is put together from discontinuous wires that are soldered somewhere in the middle (wire colors don't match on both ends) - really bad idea as solder joint can break/fracture, corrode and start acting up (more so as the engine bay warms up). I still don't want to rip it apart but will do so if pressed. Also bad thing is that I could not find replacement at the junkyard - bunch of Focuses but all had different connector (main harness plug) even though the engine was the same (Zetec).

For now I zip-tied "injector rail" to fuel rail and the FPS connector (with broken clip) is tied to FPS. Tomorrow I test drive it on the way to work.
Weekend will be for harness work, I may get replacement FPS sensor at junkyard (to get rid off zip tie on this connector).

PCV valve - I will get one once I dealt with harness. What grease are you referring to? I could not find nothing other than dielectric grease (but I do see some grease on contacts). I did clean and air dusted all connections, never seemed to be really contaminated.

Little late but I've promised an update.
After more than week and ~500 miles since applying zip-tie fix to fuel pressure sensor plug I can say that the issue is gone. The vehicle is pleasure to drive - as much as economy car can be, now if only traffic was better.
Visited local junkyard and secured FPS and pigtail (plus other pieces for total of 16$ - better than Internet prices) but for now I'll keep zip-ties - I'm not cutting into the wires unless I have a good reason to.

Now to sum up the ordeal - I'm not sure who messed up the connector/plug on FPS - could be me, could be repair shops (during unnecessary but suggested repairs they had to disconnect it at more than one occasion). It's quite likely that the issue started due to normal wear (spark plugs/wires/cables) but later on it switched to FPS issue as the connector got damaged. What really sucks is that none of the shops I took the vehicle to was capable of correct diagnostics. Instead they relied on hunch (which is OK if you're an amateur trying to fix your own car "on cheap") and suggested quite pricey repairs (some of which required pulling apart stuff that's better to keep stock).

Lesson learned - take it to the shop only for things that are regular maintenance procedures you can't do at home. Don't really expect finding out the root cause of non-obvious issue (unfortunately you have to live with it or end up paying for nothing).

The reason I suggested plugs was for diagnosis purposes because within a couple of weeks you'd have deposits to look at that would tell you if you had a problem with one cylinder. You won't get that with rare metal plugs- only standard nickel steel plugs.

As far as replacing the wiring goes- yeah, I'd do what you're doing. The grease I was suggesting is di-electric grease. You can get it in small packets at some parts stores labeled as "spark plug boot grease". That just helps prevent dust and liquid infusion from a weak connection.

When you go to solder, here's my method. Strip off about 3/4" of wire from one wire, on the other wire strip off a little more than twice as much as you stripped from the first one. Twist the strands together on both wires to help you work with them. Slide your shrink wrap insulation repair over one of the wires and far enough back not to be damaged by the soldering iron. Be sure to use at least 6". This stuff is available at most hardware stores. Now, turn 3/4" of the long stripped wire up at a 90. Match that end with the other wire and twist these 2 together. It's a little tedious to explain, but don't just twist the wires by grabbing the ends and turning each together. Fold them out like helicopter blades and make sure that each is wrapping 360 around the other one on each turn making a sort of braid. Now fold that twist back on the remaining stripped wire. Place your soldering iron under this joint and heat the wire until it melts the solder. Slide the shrink insulation over the joint after it cools off, and heat it with a hair dryer. It's a little bumpy, but if done properly you'll never have a failure. I used to use this method without solder to install stereos. The connections remained good forever, and the bonus was that if you pulled hard enough on the wires, you could pull them apart. It takes a good bit of strength though. If it's soldered, then not even that will pull apart. Making the right sort of twist is the key, so I hope I explained that correctly. And yes, I'd do it without solder myself. The shrink wrap insulation alone will prevent the wires from pulling apart in a twist/fold joint- at least until the point where the copper wire would break anyway.

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